1. Field of the Invention
The present invention relates to control of DC-DC converter, and more particularly to control at light load in DC-DC converter of synchronous rectifying system.
2. Description of the Related Art
A personal digital assistant operates on battery power, but along with power consumption by continued operation of the device, the battery power is consumed and the battery output voltage declines with the passing of the time. In order to maintain a constant voltage level of the device power source in spite of such time course changes of the battery voltage, the supply voltage is stabilized by a DC-DC converter.
The DC-DC converter used in the personal digital assistant is particularly required to be high in the efficiency of power conversion. It is necessary to minimize the power loss in order to extend the operating time of the device until the battery is recharged or replaced. The DC-DC converter of synchronous rectifying system comprises a synchronous switching element which conducts in synchronism with the timing of discharging the electric power accumulated in the choke coil to the load side. The synchronous switching element is a switching element having a rectifying action which is usually non-conducting, and establishes a current path by conducting at the timing of passing the discharge current to the load. As compared with ordinary rectifying element such as diode, it is intended to decrease the conduction loss.
When supplying power to the device by converting the battery voltage, a high efficiency is demanded in a wide load range from heavy load of large power supply to light load of small power supply. In this case, the power conversion efficiency tends to drop significantly at light load as compared with the time of heavy load. It is because the rate of power consumption of the DC-DC converter itself in the entire power consumption is relatively large.
To improve decline of power conversion efficiency at light load, a DC-DC converter of PFM (pulse frequency modulation) system is proposed. The PFM system is a bypass method of switching action depending on the output voltage of the DC-DC converter. By decimating the switching actions while the supply of load power is provided enough by the accumulated power of the output capacitor connected to the output terminal of the DC-DC converter, the power loss of the DC-DC converter is decreased, and it is intended to improve the power conversion efficiency at light load.
In the PFM system, however, the switching period varies depending on the load power, and it may be noise source for the device and adverse effects may be considered. Accordingly, a DC-DC converter of PWM (pulse width modulation) system is proposed, which is intended to perform switching action at a specific period in synchronism with the clock signal oscillated at a specific frequency. Because the switching action is done at specific frequency, it is effective for devices likely to have adverse effects by fluctuations of switching frequency.
Another DC-DC converter is proposed for controlling by changing over the PFM system operation and PWM system operation depending on the load. An example of this technology is disclosed in Japanese Laid-open Patent No. H6(1994)-303766. In ordinary operation including heavy load, the operation is based on the PWM system, and at light load, it is changed over to the PFM system. As a result, while improving the power conversion efficiency at light load, adverse effects on the device in ordinary operation can be decreased. In Japanese unexamined patent publication No. H6(1994)-303766, the switching action is controlled by fixing the OFF time specifically, but in the nearly fixed state of voltage values of input voltage and output voltage, the switching duty is nearly specific, and the switching action can be done nearly at specific frequency. Therefore, the switching action control at specific OFF time may be also regarded as PWM system.